The present invention relates to a method and apparatus for measuring magnetic losses in ferromagnetic materials.
Ferromagnetic materials are used in a large number of electrical devices functioning with alternating current. Two main applications are for transformers and electrical motors. An important consideration in the choice of ferromagnetic material for such applications is the level of magnetic loss which occurs as a result of eddy currents and magnetic hysteresis. For this reason, much effort has been devoted over the years to the development of low loss ferromagnetic materials such as oriented Fe-3%Si steel and more recently amorphous magnetic materials. Many of these materials are presently sold with specified magnetic losses usually rated for a given frequency and peak magnetic induction.
Magnetic losses are generally measured using electromagnetic measurements. The standard technique used in the silicon steel industry is based on placing 3 cm wide by 25 cm long strips of the sample in a standard Epstein frame (ASTM standard) which includes coils for generating a magnetic field and coils for measuring the resulting magnetic induction in the sample. In this measurement, the exiting magnetic field H is deduced from the current and number of turns in a primary circuit of the Epstein frame and the magnetic induction B is deduced from a secondary coil also wound around the sample.
Magnetic losses can also be measured on-line, for example during fabrication of large strips of ferromagnetic material, by using suitable coils to produce a magnetic field H in the material and to measure the magnetic induction B. These techniques are rapid, accurate, sensitive and well accepted in the industry. However, they have some limitations:
(1) they require good simultaneous measurements of both the magnetic field H and magnetic induction B, which necessitate in some cases (e.g. for on-line measurements) complicated coil geometries; PA1 (a) applying to the specimen an alternating magnetic field having a predetermined frequency and an amplitude which is modulated at a frequency lower than the predetermined frequency; PA1 (b) measuring a temperature modulation at a surface of the specimen having the applied field; and PA1 (c) determining the magnitude of magnetic losses in the specimen from the measured surface temperature modulation. PA1 magnetizing means for generating an alternating magnetic field having a predetermined frequency and an amplitude which is modulated at a frequency lower than the predetermined frequency, and applying such a magnetic field to the ferromagnetic material specimen, and PA1 temperature sensing means for measuring a temperature modulation at a surface of the specimen having the applied field, whereby the magnitude of magnetic losses in the specimen is determined from the measured surface temperature modulation.
(2) since it is necessary for the coils to surround the material undergoing measurement, magnetic losses are usually an average over the entire coil area and it is next to impossible to measure losses locally, for instance near edges or at the center of a large plate.